CF10/LV overcomes acquired resistance to 5-FU/LV in colorectal cancer cells through downregulation of the c-Myc/ABCB5 axis.

Acquired resistance to 5-fluorouracil/leucovorin (5-FU/LV) frequently develops during treatment of metastatic colorectal (mCRC), but the causes are incompletely understood. We aim to: (i) identify the causes of 5-FU/LV resistance under physiological folate; and (ii) determine if a polymeric fluoropyrimidine (FP) CF10 remains potent to CRC cells selected for 5-FU/LV resistance. 5-FU/LV-resistant CRC cells were selected by repeated passaging with increasing 5-FU/LV concentrations, and resistance factors were calculated from dose-response studies. Basal and treatment-induced thymidylate synthase (TS), Myc, and ABCB5 were determined by RT-qPCR and Western blot. TS activity was determined using an H-release assay. DNA topoisomerase 1 cleavage complexes (Top1cc) and DNA double-strand breaks (DSBs) were determined by immunofluorescence. Acquired resistance to 5-FU/LV with physiological folate was associated with a <1.5-fold increase in basal TS levels; however, with either 5-FU/LV or CF10/LV treatment, TS levels were elevated ~5-fold by Western blot but only ~2-fold by RT-qPCR. CF10 remained very potent to CRC cells selected for 5-FU/LV resistance, and CF10 effectively induced TS ternary complex formation and inhibited TS catalytic activity in 5-FU/LV-resistant CRC cells. c-Myc was expressed at ~4-fold higher levels in 5-FU/LV-resistant CRC cells, but Myc was barely detectable with CF10/LV treatment. The Myc-target ABCB5, which is an established factor in resistance to 5-FU and other drugs, was substantially downregulated with CF10/LV but not 5-FU/LV treatment. Acquired 5-FU/LV resistance was associated with FP-induced TS and elevated Myc and ABCB5. There is minimal cross-resistance to CF10 in 5-FU/LV-resistant CRC cells, consistent with its use in treating 5-FU/LV-resistant mCRC.